“Spinel-layered” Li_1+xNi_0.5Mn_1.5O₄ (x = 0, 0.5, 1) materials are considered as a cobalt-free alternative to currently used positive electrode (cathode) materials for Li-ion batteries. In this work, their electrochemical properties and corresponding phase transitions were studied by means of synchrotron X-ray powder diffraction (SXPD) in operando regime. Within the potential limit of 2.2–4.9 V vs. Li/Li⁺ LiNi_0.5Mn_1.5O₄ with cubic spinel type structure demonstrates the capacity of 230 mAh·g⁻¹ associated with three first-order phase transitions with significant total volume change of 8.1%. The Li₂Ni_0.5Mn_1.5O₄ material exhibits similar capacity value and subsequence of the phase transitions of the spinel phase, although the fraction of the spinel-type phase in this material does not exceed 30 wt.%. The main component of Li₂Ni_0.5Mn_1.5O₄ is Li-rich layered oxide Li(Li_0.28Mn_0.64Ni_0.08)O₂, which provides nearly half of the capacity with very small unit cell volume change of 0.7%. Lower mechanical stress associated with Li (de)intercalation provides better cycling stability of the spinel-layered complex materials and makes them more perspective for practical applications compared to the single-phase LiNi_0.5Mn_1.5O₄ high-voltage cathode material.

In the present work, we focus onthe experimental screening of selected electrolytes, which have been reported earlier in different works, as a good choice for high-voltage Li-ion batteries. Twenty-four solutions were studied by means of their high-voltage stability in lithium half-cells with idle electrode (C+PVDF) and the LiNi_0.5Mn_1.5O₄-based composite as a positive electrode. Some of the solutions were based on the standard 1 M LiPF₆ in EC:DMC:DEC = 1:1:1 with/without additives, such as fluoroethylene carbonate, lithium bis(oxalate) borate and lithium difluoro(oxalate)borate. More concentrated solutions of LiPF₆ in EC:DMC:DEC = 1:1:1 were also studied. In addition, the solutions of LiBF₄ and LiPF₆ in various solvents, such as sulfolane, adiponitrile and tris(trimethylsilyl) phosphate, atdifferent concentrations were investigated. A complex study, including cyclic voltammetry, galvanostatic cycling, impedance spectroscopy and ex situ PXRD and EDX, was applied for the first time to such a wide range of electrolytesto provide an objective assessment of the stability of the systems under study. We observed a better anodic stability, including a slower capacity fading during the cycling and lower charge transfer resistance, for the concentrated electrolytes and sulfolane-based solutions. Among the studied electrolytes, the concentrated LiPF₆ in EC:DEC:DMC = 1:1:1 performed the best, since it provided both low SEI resistance and stability of the LiNi_0.5Mn_1.5O₄ cathode material.